An MRAM (Magnetoresistive Random Access Memory) is a magnetic memory which utilizes the reversal of magnetization. A spin-transfer MRAM, which uses write by spin transfer, is excellent in high speed performance, high integration density, and durability, and is prospective as a general-purpose nonvolatile random access memory.
In the spin-transfer MRAM, an MTJ (Magnetic Tunnel Junction) element is used as a memory element. This MTJ element includes a storage layer including a magnetic layer, the direction of magnetization of which is variable by a write operation of the memory; a reference layer including a magnetic layer, the direction of magnetization of which is fixed in one direction; and a tunnel barrier layer which is interposed between the storage layer and the reference layer and forms a tunnel barrier. Depending on whether the magnetization of the storage layer and the magnetization of the reference layer are in a parallel state or in an antiparallel state, the electrical resistance in a case of causing an electric current to flow in a direction perpendicular to the film surface of the MTJ element takes a low resistance state or a high resistance state. By using a difference in resistance between the parallel state and the antiparallel state, data (information) can be read from the MTJ element.
In the write by spin transfer, the magnetization of the storage layer is reversed by causing an electric current to flow in a direction perpendicular to the film surface of the MTJ element. For example, when the magnetization is reversed from the antiparallel state to parallel state, an electric current is caused to flow in such a direction that electrons flow from the reference layer to the storage layer. The direction of the electric current becomes, conversely, a direction from the storage layer toward the reference layer. By this current flow, a spin torque acts on the magnetization of the storage layer such that the magnetization of the storage layer becomes parallel to the magnetization of the reference layer, and the magnetization of the storage layer can be reversed by causing a current of a predetermined threshold or more to flow. On the other hand, when the magnetization is reversed from the parallel state to antiparallel state, an electric current is caused to flow in such a direction that electrons flow from the storage layer to the reference layer. By this current flow, a spin torque acts on the magnetization of the storage layer such that the magnetization of the storage layer becomes antiparallel to the magnetization of the reference layer. In this manner, by changing the direction of the electric current that is caused to flow, data rewrite is enabled.
In the MRAM using spin-transfer writing, an electric current is applied to the MTJ element through the same path at a time of read and at a time of write. Thus, there is, potentially, a risk of read disturb by which data is rewritten at a time of read. In order to avoid this risk, there is a method of setting a read current, which is supplied to the MTJ element at a time of read, to be lower than a write current which is supplied to the MTJ element at a time of write. By this technique, the probability of occurrence of read disturb is reduced. However, to decrease a read current causes a decrease in read sensitivity. Thus, there is a lower limit to a practical read current.
This being the case, in order to avoid the occurrence of read disturb, such a method has been proposed that the probability of occurrence of read disturb is reduced by setting the pulse width of a read current to be smaller than the pulse width of a write current. However, in a memory which requires a high speed operation, the pulse width of a write current becomes smaller due to the demand for an increase in speed of the write operation. It is thus necessary to make smaller the pulse width of the read current, but there is also a lower limit to the pulse width of the read current, because of problems of read sensitivity and wiring delay of current pulses.
Furthermore, it has been reported that the write current increases if the pulse width of the pulse of the write current is decreased in order to meet a demand for an increase in speed at a time of write. Thus, as regards the reduction in write power, that is, power saving, an importance is placed on the reduction in write current.